The present invention relates to a heater control device which controls a heater used in an image forming apparatus such as a laser printer, a copying machine, or a facsimile machine, which performs operations for forming an image. More particularly the present invention is directed to a heater control device which can be used in any image forming apparatus regardless of the differing voltage levels of available commercial power provided in various geographic regions of the world.
For example, a laser printer, which records character and picture images by forming electrostatic latent images on a photoreceptor, develops such electrostatic latent images thus formed by applying toner and thereafter transfers the toner images onto printing paper. The laser printer typically fuses the toner image on the printing paper by a heating process using an electric heater such as a heating roller.
An image forming apparatus typically includes electrical component parts, such as transistors and integrated circuits (ICs), which are driven with a direct current power source, and component parts, such as the heater mentioned above, which are driven with an alternating current drive source. Motors and exhaust fans which are used as power sources have been customarily driven with an alternating power source, but have recently come to be driven quite frequently with a direct current power source. This tendency is due to the fact that a direct current power source can achieve higher accuracy in the control of these electrical component parts and also that it can secure the same characteristics of such electrical component parts in geographic regions where the frequency of the commercial power source is 50 Hz, for example, and in geographic regions where the frequency of such a power source is 60 Hz.
In image forming apparatuses, it is frequently found that an alternating current continue to be used as they are even at the present time for heaters and some electrical component parts for such reasons as their consumption of a relatively large amount of electric power and the possibility of their control with alternating current.
In this regard, various kinds of industrial products have come to be distributed extensively throughout the world in recent years, and such products manufactured in one country are often exported to many different countries. Also, it happens quite frequently that such a manufactured product, particularly a small-sized product, is used in a number of countries when its possessor travels from country to country. Under these circumstances, the differences in voltage of the commercial power sources in the individual countries in the world have begun to present problems. For example, the commercial power source in Japan is 100 volts, whereas the voltage of the commercial power source is 115 volts or 120 volts in many areas in the United States, and in the range from 220 to 240 volts in many areas in the Near and Middle East regions, Africa, and Europe. Additionally, it is not rare that the voltage of the commercial power source is different among neighboring countries, and, in fact, there are actual cases in which the voltage of the commercial power source is different among various regions in a single country.
Even under these circumstances related to the voltage of the commercial power sources, the electric products which work with battery cells as their power sources do not require any modification to their power sources for use in different regions. Moreover, the electrical component parts, such as integrated circuits (ICs) and direct current motors, which are operated with direct current power sources, are designed to work with a power source voltage, such as 24 volts or 5 volts, do not therefore require any direct modification to their power source on any region-by-region basis.
However, the electric component parts, such as a fusing unit mentioned above, which are operated with an alternating current power source, cannot be used in common in areas in which the voltage of the available commercial power source is considerably different. For example, if a heater designed for operation at the power source voltage of 100 volts is used in an area where the voltage of 200 volts is used, the heater, switch, and other electric parts will be destroyed by the energy of the excessive current, and some damage to the circuit parts occurs due to the high temperature which results. To the contrary, if a heater designed for operation at the power source voltage of 200 volts is used in an area where the voltage of 100 volts is used, it will not be practical to use the heater since it may suffer from a decline in its ability to generate heat, with the result that the heater fails produce sufficient heat or takes an exceedingly long period of time before it can satisfactorily perform any fusing work.
Therefore, it has hitherto been in practice to make alternating current electrical component parts, such as heaters, which are suitable to the voltage of each commercial power source and to use such component parts selectively in a manner fitting to the voltage of the particular commercial power source. In this situation, it has been necessary until now to manufacture many kinds of electrical component parts to be operated with alternating current for ensuring their compatibility with the voltages in various countries, even though the electrical components for operation with direct current are used in common among the products made for use in different countries. Thus, each image forming apparatus had to be manufactured differently for its exportation to various countries in the world. As a result, there have been problems as relatively high costs of such component parts and considerable difficulties in keeping many different types of component parts in stock. Furthermore, heaters are constructed in such a manner that they are covered with a quartz pipe or the like so that they may be used at as high a temperature as possible, and yet heaters have been faced with the problem that the pipe surface of such a heater is liable to breakage when a person other than a service engineer inadvertently touches it. Thus, it has not been a desirable practice even for a service engineer to take such a heater along with him to deal with a change in the voltage of the power source.
With a view to dealing properly with this problem, there has been proposed a technique for reducing the voltage of the power source to one half by using a transformer in case the voltage of the power source is 200 volts and a technique for providing two heaters connected in parallel in case the voltage of the power source is 100 volts or two heaters connected in series in case the voltage of the power source is 200 volts, so that the power consumption is made equal without any dependence on the voltage of the power source. These techniques are disclosed in in Japanese Patent Unexamined Publication No. Hei. 3-75805.
However, the technique for reducing the voltage of the power source to one half by using a transformer requires a transformer as well as a power relay for switching the input terminals for the primary winding in the transformer for a changeover between the time when the voltage of the power source is 100 volts and the time when it is 200 volts. This requirement results in an increase cost and a larger size of the apparatus.
Also, the technique for switching between the series connection and the parallel connection of two heaters entails an increase in the number of heaters and the number of heater connectors needed for connecting the heaters with the power source, etc. This technique also requires a power relay for switching the heaters between a series connection and a parallel connection and consequently also results in an increase of in the cost and size of the apparatus.
Moreover, it is necessary for an image forming apparatus, such as a laser printer, to maintain the temperature of the heating roller with a built-in heater at an approximately constant temperature in order to secure uniform image quality. For this reason, it has been in practice in the past to perform control with a temperature detecting device in such a manner that the power conducted to the heater is cut off when the detected temperature has risen to or; above a preset temperature, but that the power is conducted again to the heater when the detected temperature has fallen below the preset temperature. Yet, such a control presents the problem that violent fluctuations occur in the temperature of the heating roller. Above all, this control causes the problem that an overshoot occurs at the time during which the apparatus warms up.